1. Field of the Invention
The present invention relates to a reflecting triple grating type optical encoder which detects a position of an object to be detected.
2. Description of the Related Art
An optical encoder which includes a detecting head and a scale are hitherto known (refer to Japanese Patent Application Laid-open Publication No. 2003-166856 for example). FIG. 21 shows an example of a structure disclosed in Japanese Patent Application Laid-open Publication No. 2003-166856. The detecting head includes a light source 14 in which a first grating 11 is disposed, and a photodetector 50 in which a third grating 30 is disposed. Moreover, a second grating 20 is formed on a scale 21. Light emerged from the light source 14 forms a self image on a light receiving surface of the photodetector 50 upon being affected by the first grating 11 and the second grating 20. In this patent application, “self image” means an optical pattern having a shape substantially similar to the second grating 20 formed on the light receiving surface of the photodetector 50. Moreover, the self image moves according to a relative displacement of the detecting head and the scale 21. The photodetector 50 detects the movement of the self image. Such type, hereinafter, will be called as a “reflecting triple grating type”.
In the reflecting triple grating type optical encoder, it is desirable to perform a detection which is not affected easily by a change in a distance between the scale and the detecting head. For this, the first grating and the third grating included in the detecting head are disposed by lining up the heights thereof. Moreover, the self image can be classified into the following two typical cases (a) and (b) for example, according to structural conditions of the optical encoder, where
case (a) is a case of having a same space period as of a shadow pattern of the second grating, and
case (b) is a case of having the space period ½, ¼, and . . . of the shadow pattern of the second grating.
The most stable and the clearest image is formed in the case (a) mentioned above. Therefore, the case (a) mentioned above will be described below.
When the following conditional equation (1) is satisfied, a self image having the same space period as of the shadow pattern of the second grating is observed on the light receiving surface of the photodetector.1/z1+1/z2=λ/(k×p22)  (1)where,                λ denotes a central wavelength of light emitted from the light source,        z1 denotes an optical distance between the first grating and the second grating,        z2 denotes an optical distance between the second grating and the third grating,        p2 denotes a pitch of the second grating, and        k denotes a natural number.        
Here, when                Δz0 is let to be a difference between z2 and z1, which is designed in advance,        Δz is let to be a difference between z2 and z1 when actually manufactured, and        Δzd is let to be a difference between Δz0 and Δz which occurred when actually manufactured (in other words, a difference between Δz0 designed in advance and Δz when actually prepared) the following equation (2) holds.Δz=Δz0+Δzd  (2)        
Inventors of this patent application performed the following experiment, and calculated a tolerance of Δzd required for achieving satisfactory signal strength by the photodetector. According to results of the experiment, it was revealed that, with predetermined conditions, to achieve the satisfactory signal amplitude at the photodetector, such as signal strength at which the maximum signal amplitude is ½ for example, the tolerance of Δzd is ±36 μm. Moreover, it was revealed that with other conditions, for achieving the satisfactory signal strength at the photodetector, the tolerance of Δzd is approximately ±33 μm. Generally, the tolerance of machining accuracy is in a range of ±50 μm to ±100 μm. Therefore, from a point of view of manufacturing, it is extremely difficult to dispose by mounting the first grating, the second grating, and the third grating such that the Δzd is in a range of ±36 μm to ±33 μm. Moreover, when an attempt is made to realize an optical encoder in which Δzd is not greater than the range of ±36 μm to ±33 μm, a dimensional tolerance of structure members of the optical encoder being small, it becomes very difficult to manufacture at a low cost. Moreover, in the conventional example described above, a concrete method or a structure which is assembled with the tolerance Δzd to be very small is not at all disclosed.